1. Field of the Invention
This invention relates to sensors used for on-line monitoring of the state (condition) of high voltage insulation in electrical equipment with capacitance (potential) taps and the interconnection therewith to remote measuring devices. The electrical equipment may include bushings of power transformers, shunt reactors or circuit breakers and current transformers.
2. Description of Prior Art
The On-line monitoring of high-voltage insulation of electrical equipment is performed on the equipment under operation, i.e. in the actual operating condition. Equipment de-energization is required only for the initial sensor installation. As increasingly reliable and cost- and labor-effective, this technology is now widespread in numerous applications. Particularly, such monitoring provided concurrently on power frequency and radio frequencies may be an effective tool in prediction and prevention of in-service failures for high-voltage bushings and other equipment with capacitance (potential) taps.
Attention is called to the following Publications:
xe2x80x9cMethods and Tools for High-Voltage Equipment Diagnosticsxe2x80x9d, Energoatomizdat Publishing House, Moscow, by P. Svy 1992.
xe2x80x9cExperience in the Application of the On-Line Monitoring System Using Power Frequency and Partial Discharges to High Voltage Transformer and Bushing Insulationxe2x80x9d, by Z. Berler, L. Letitskaya and P. Svy, EPRI Substation Equipment Diagnostic Conference VI, Feb. 16-18, 1998, New Orleans, La.
Bushings of power transformers, shunt reactors or circuit breakers and current transformers, with their internal insulation of oil-impregnated paper similar to that used in cables or capacitors, are equipped with so called capacitance or potential taps. A capacitance tap is connected to a metal foil shield inserted inside the insulation. The insulation has certain capacitance and conductance between the high voltage current-carrying conductor and the foil shield. Both the capacitance value and the power factor of the insulation depend upon the insulation condition and could be quantified at the tap output with the equipment on-line. Furthermore, the electrical impulses that accompany partial discharges inside the insulation are also coupled to the output of the capacitance tap and can be detected using circuits of a suitable design.
The capacitance taps were originally designed only for relatively rare off-line insulation tests using a suitable test source at power frequency. During equipment operation they remained grounded. It was recognized that these taps lend themselves as excellent means of on-line monitoring of the insulation. The use of the capacitance tap for an on-line monitor requires a sensing device to be inserted permanently between the live tap contact and the ground. The aforementioned publications teach such an arrangement.
The sensor designed for the power frequency measurement produces a signal proportional to the capacitive current through the bushing insulation. The sensor designed for partial discharges senses the radio frequency impulses and produces a signal of magnitude proportional to the dissipated electrical charges. The repetition rate of such discharges can be determined by a measuring device.
Sensors based on application of current transformers are described in U.S. Pat. No. 5,471,144 xe2x80x9cSystem for Monitoring the Insulation Quality of Step Graded Insulated High Voltage Apparatusxe2x80x9d issued Nov. 29, 1995; U.S. Pat. No. 5,574,378 xe2x80x9cInsulation Monitoring System for Insulated High Voltage Apparatusxe2x80x9d issued Nov. 12, 1996; U.S. Pat. No. 5,640,154 xe2x80x9cInsulation Monitoring System for Insulated High Voltage Apparatusxe2x80x9d issued Jun. 17, 1997; and U.S. Pat. No. 5,652,521 xe2x80x9cInsulation Monitoring System for Insulated High Voltage Apparatusxe2x80x9d issued Jul. 29, 1997 and in the Svy reference, P. 107. They consist of a current transformer with a primary winding created by the capacitance tap grounding conductor, and a secondary toroidal winding consisting of several or many turns. This current transformer can be coreless (so-called Rogovsky coil), as suggested in the above mentioned patents for power frequency measurements, or with a ferrite core, as recommended in the Svy Reference for the radio frequency impulse measurements. The advantage of the current transformer-based sensor is its simplicity. A current transformer with its secondary winding loaded with a small resistance has small input impedance, so there is usually no need for a special tap overvoltage protection.
Monitoring of radio frequency (partial discharge) impulses imposes different requirements on sensor design, as opposed to monitoring of signals at power frequency. For partial discharge monitoring it is desirable to detect a frequency band generally between 0.5 and 20 MHz with high sensitivity. Ferrite radio frequency transformers with a small number of turns in the secondary winding are appropriate for this task as they are capable of accurately transmitting short and steep pulses, but they block power frequency signal. A coreless current transformer with a large number of turns in the secondary winding can be employed for power frequency measurement, but it is practically insensitive to weak partial discharge pulses. To meet both requirements, two separate transformers, one of each type, are necessary.
A coreless Rogovsky coil has a low sensitivity even at the power frequency signals. For this reason it was replaced with a resistor shunt connected between the output of the tap and local ground (Russian Patent 292,062, published Feb. 12, 1971). The measured quantity, a power frequency voltage drop across the resistor shunt, is directly proportional to the capacitive current through the bushing insulation. The magnitude of the power frequency signal can be conveniently controlled by the resistance chosen for the shunt. The disadvantage of such an arrangement is that the tap capacitance, between the high voltage line and the output of the capacitance tap, in series with the resistance of the sensor shunt represents a frequency dependent voltage divider. As a result, switching and lightning transients can cause severe overvoltages at the output of the tap due to their very high frequencies. These transients have the potential of destroying not only the measuring circuit, but also the insulation of the tap output or even the bushing. To limit the transients, a surge arrestor is added in parallel to the resistor shunt, as shown in the Svy Reference, on its FIG. 8.2.
A further improvement of the sensor consisted of replacing the resistor shunt with another capacitor, see U.S. Pat. No. 4,757,263 xe2x80x9cInsulation Power Factor Alarm Monitorxe2x80x9d issued Jul. 12, 1988; U.S. Pat. No. 5,903,158 xe2x80x9cMonitoring of Internal Partial Discharges in a Power Transformerxe2x80x9d issued May 11, 1999; and U.S. Pat. No. 6,028,430 xe2x80x9cMethod for Monitoring a Capacitor Bushing, and Monitoring Systemxe2x80x9d issued Feb. 22, 2000. This arrangement features a capacitor divider ratio that is essentially independent of frequency, thus minimizing the exposure of the tap and the low voltage circuits to destructive switching and lightning impulses. A surge arrester is kept in place as a xe2x80x9csecond line of defensexe2x80x9d for rare cases of extremely severe overvoltages.
All of the sensor designs described above are mutually exclusive in that they can satisfy only one application at a time; a power frequency signal detection or a partial discharge detection, but not both. With only one capacitance tap available per bushing, this represented a serious disadvantage as the replacement of a bushing sensor requires outage.
A Publication entitled xe2x80x9cOn-Line Monitoring of Power Transformer-Trends, New Developments and First Experiencesxe2x80x9d by T. Leibfried, W. Knorr, K. Viereck, CIGRE, 1998, #12-211, teaches a sensor that can contain both circuits. The sensor relies on the capacitor shunt connected to the tap output and the radio frequency current transformer the primary winding of which is connected in series with the capacitor shunt, either on its grounded side or on its xe2x80x9clivexe2x80x9d side. Two separate coaxial cables carry power frequency and radio frequency signal signals respectively. Similar sensors were used by Cutler-Hammer starting in 1996.
These sensors have disadvantages. It is the necessity to use two cables to carry the information extracted from a sensor. Another disadvantage is that, compared with the sensor that utilizes a current transformer only, this sensor has lower sensitivity to partial discharge impulses: on high frequencies the stray capacitance of the surge arrester shunts the circuit of series connected radio frequency current transformer and capacitor shunt, thus diverting part of high frequency current from entering into current transformer.
In accordance with the invention, a partial discharge determination system for an electrical system which includes: a conductor at a given voltage potential, electrical insulation disposed proximate the conductor and an insulator capacitance disposed in the insulation which conducts partial discharge electrical current is taught. The partial discharge electrical component may be random and occasional. It has a radio frequency impulse associated therewith. A steady state generally continuous power frequency current component may flow in parallel at the same time. There is a sensor capacitor shunt, the primary winding of the sensor current transformer is connected electrically to the insulator capacitance to conduct partial discharge electrical current there through. There is also a coaxial cable, the coaxial cable is connected to the sensor current transformer primary winding. There is also present a monitoring power frequency capacitor. A monitoring radio frequency isolation transformer is also present. A monitoring choke coil is present. The monitoring choke coil and the monitoring power frequency capacitor are interconnected electrically with the coaxial conductor. There is a monitoring surge arrester. The monitoring choke coil, the monitoring surge arrester and the monitoring power frequency capacitor are interconnected electrically with the coaxial conductor. A first signal representative of an electrical power frequency current component exists between the monitoring choke coil and the monitoring surge arrester. A second signal representative of the radio frequency current associated with partial discharge component exists between the monitoring radio frequency isolation transformer and the monitoring radio frequency isolation transformer secondary winding second end.